This invention relates generally to apparatus and methods for providing a linearized analog signal proportional to the watercut of an emulsion and more particularly, but not by way of limitation, to such a device and method utilizing a programmable read only memory containing digital values which effectively linearize the non-linear response of a capacitance probe to the percent of water in the monitored emulsion.
The liquid produced from an oil well is an emulsion when the liquid includes both oil and water, which is the case for production obtained from a well produced by a water flood operation, for example. Some of the water may be separated from the emulsion by flowing the emulsion through a separator; however, the stream from the outlet of the separator is generally still an emulsion because it has some water mixed with the oil. To determine how much water remains--which is desirable information to have because it enables one to determine whether further separation should be performed, for example--a watercut (percentage of water) measurement is made.
A general technique known to us for making a watercut measurement uses a capacitance probe yielding an electrical capacitance non-linearly variable in response to changes in the percentage of water in the emulsion. The technique of measuring the watercut with a capacitance probe is well known. See, for example, Journal of Petroleum Technology, November, 1962, "BS&W Measurements Principles and Practices", W. J. Warren and Journal of Petroleum Technoloqy, October, 1968, "Digital Instrumentation for Net Oil Measurement in Well Testing", Carl W. Zimmerman et al. This technique using a capacitance probe allows one to determine the proportions of an oil-water flow without requiring the actual physical separation of the two liquids.
Equipment for making a watercut measurement with a capacitance probe is or has been made or offered by companies including Halliburton Services (a division of Halliburton Company), Hydril Company, Baker Production Services, C. E. Invalco, General Manufacturing Company, and Engelman-General, Inc. This equipment typically provides a signal from a watercut monitor device which includes the capacitance probe; it also provides a signal from a flow meter; these two signals are provided to a computer or analysis device located in a housing separate from the watercut monitor device and the flow meter; the housing has displays visually showing the watercut and the totals of oil and water computed on the basis of the information derived from the signals from the watercut monitor and the flow meter. One or more of the watercut monitor devices are believed to use operational amplifiers and filter circuits to provide a linear analog output. One or more are believed to use frequency-to-voltage conversion or analog integration techniques to compensate for the non-linear responses of the capacitance probe; in at least one of these types of equipment, linearization is done in the readout device under microprocessor control using a look-up table of linearized values. Such analog circuits require retrimming or redesign to accommodate changes in the linearization or in the characteristic of the output signal from the watercut monitors (e.g., does full-scale output indicate 100% watercut or something less). As to at least the digital linearization performed under microprocessor control, this is done in a different unit from the watercut monitor device. In general, these types of equipment have been provided as complete systems to be used intact by customers.
Increasingly, however, there has been a demand in the oilfield for instrumentation which will enable the information derived from the primary sensor, such as the watercut monitor including the capacitance probe, to be provided as an analog signal having a desired characteristic linearly related to the detected watercut so that such signal can be transmitted directly to a central computing device, thereby obviating the need for the intermediate analysis and readout devices which have heretofore been used. Therefore, there is the need for an improved watercut monitor device and method which linearize the non-linear capacitance probe output by strictly digital techniques to facilitate maintenance and modification, but which ultimately provide a linearized analog signal as an output for use by other equipment. Such improved device and method would enable users to have a compatible linear analog signal for use in their own analysis equipment.